1. Field of the Invention
The present invention relates to an air intake system for a supercharged engine.
2. Description of the Related Art
Heretofore, various engines of a type capable of supercharging intake air by superchargers in order to increase the quantity of the intake air for the engine are known to those skilled in the art, as disclosed for example in Japanese Utility Model Unexamined Publication (kokai) No. 171,630/1981. Further, there are known a turbo supercharger of a type driven by exhaust gases and a mechanical supercharger of a type driven by an output shaft of the engine.
Conventional supercharged engines are so set as to have a geometric compression ratio of the engine at a relatively low value of less than 8.5 because knocking is likely to occur in a highly supercharged region if the geometric compression ratio is enlarged. However, if the geometric compression ratio of the engine is lowered, there may arise the tendency that cycle efficiency of the engine is reduced. Further, for a supercharged engine, it is required to insure reliability of an exhaust system in a highly supercharged region by suppressing the rise of the temperature of exhaust gases. At this end, such conventional supercharged engines are so designed as to reduce the temperature of the exhaust gases by making the air/fuel ratio lean to a rich range in a high-speed, high-load operation by cooling fuel; however, this requires more fuel than the quantity of fuel required for output.
Under those circumstances as described herein-above, conventional supercharged engines cannot be said to improve mileage, particularly so in a high-speed, high-load region, so that an improvement in this point has been demanded.
A copending Japanese patent application (Japanese Patent Unexamined Publication (kokai) No. 239,312/1985) proposes a system in which the geometric compression ratio is set to be greater than conventional supercharged engines and the intake valve closing timing is delayed as compared with those conventional supercharged engines, thereby suppressing the effective compression ratio to an appropriate level while gaining the expansion ratio and as a consequence maintaining anti-knock performance to an appropriate level. This proposed system can also prevent the excessive rise of the temperature of exhaust gases without relying upon leaning the air-fuel ratio to a rich region, thereby improving reliability of the engine and fuel economy and further achieving an improvement in fuel economy due to a decrease in a loss in pumping in a low-load or middle-load region.
For such supercharged engines so designed as to delay the intake valve closing timing, the pressure within the air intake passage on the exhaust side of the supercharger rises due to the delayed closing of the intake valve, so that the engine with the mechanical supercharger has the tendency that a loss of power for driving the supercharger is increased and the volume efficiency of the supercharger is decreased. On the other hand, the engine with the turbo-charger has the tendency that the amount of air supercharged is reduced. For this reason, it is considered that such a shape of the independent air intake passage capable of achieving the inertia effect in an engine high-speed region, as widespread adopted by conventional ones so as to achieve high output is applied to the supercharged engine. Such a shape of the independent air intake passage is so arranged, for example, as to make a passage length thereof relatively longer by curving it or as to make a ratio of a transversely sectional area to the passage length thereof relatively larger. In this case, however, the pressure in the intake passage on the exhaust side of the supercharger is lowered due to the inertia effect in the engine high-speed region. For the engine with the mechanical supercharger, the loss in power for driving the supercharger is reduced because of a decrease in the back pressure of the supercharger, and a ratio of the air pressure on the intake side of the supercharger to the air pressure on the exhaust side thereof is reduced, thus leading to improving the volume efficiency for the supercharger. Further, for the engine with the turbo-charger, the quantity of air supercharged under a constant supercharging pressure, i.e. under the condition, for example, where the pressure supercharged is adjusted to be constant by a waste valve, thereby enhancing high output of the engine.
The aforesaid arrangement, however, presents the new problems that high output of the engine cannot be attained as a matter of fact in the engine high-load region and fuel economy becomes poorer as well. More specifically, those problems arise in an engine high-load, high-speed region because the decrease in the output to be caused by the adiabatic compression action resulting from the inertia effect of intake air before the intake valve closing timing is larger than the loss of its power for driving the supercharger and the increase in the volume efficiency for the engine with the mechanical supercharger, on the one hand, and than the anticipated increment of the output due to the increase in the quantity of air supercharged for the engine with the turbo-charger, on the other hand. Further, the new problem may occur due to the reduction of output and fuel economy due to retardation of the spark timing on the basis of the reduction of anti-knock performance resulting from the rise in the temperature within the cylinders, thereby consequently failing to gain the increase of the maximum power and rendering the fuel economy poorer.